A reducing agent may be defined as a substance, the oxidation number of whose atom increases. Reduction is accompanied by an increase in oxidation number of the reducing agent. Carbon belongs to IV A group. So, its maximum oxidation number is +4. In CO2 the oxidation number of carbon is +4 which cannot be further, increased. Hence, CO2 cannot act as reducing agent. While the maximum oxidation number of nitrogen, sulphur and chlorine are +5,+6 and +7 respectively. In NO2,CO2 and ClO2 the O.N. of nitrogen, sulphur and chlorine is +4. Which can be further, increased. So, they can act as a reducing agent.